HuffPost Q&A: Fishing for the Truth About Nutrition

This week, David Katz M.D., director of the Prevention Research Center at Yale University School of Medicine and HuffPost blogger, asked for your most pressing health questions. Many of you responded with personal stories about dieting, chronic illness, and genetically-modified foods. Below, Dr. Katz responded to one such question. You can submit advice questions to Dr. Katz at any time, as often as you'd like, by emailing huffpost-community@huffingtonpost.com, and your question may be featured in Dr. Katz's next blog post.

Dear Dr. Katz,

I really enjoy your blog, since you tackle issues that many people and media don't seem willing to grapple with.

I'm curious about your current position on fish. I've noticed you recommend fish, and maybe fish oil supplements, in some blog postings recently (though I can't find any specific references right now). But I've also seen recent articles and medical studies saying that no benefit to the heart or for depression, cognitive decline, or anything else have been found. I wonder if the recommendations to eat fish are cooked up or at least perpetuated by the fish producers, and maybe by some doctors based on some earlier misleading studies who think their patients wouldn't be willing to go totally plant-based but should be cutting back on meat. Obviously, one big negative of eating fish these days is the levels of mercury and other toxic chemicals (PCBs, etc.) in their flesh. Do you have any current evidence that the benefits really outweigh the disadvantages? -- Amanda Strombom, WA

While it may not wag the prevailing dietary dogma all on its own, a fish tale has long figured in it: Eating fish is good for us. Claims have gone farther, encompassing specific benefits to heart and mind in particular and invoking specific nutrient properties of fish, notably omega-3 fat.

But nothing, it seems, is entirely pure anymore -- not our food, nor the advice we get about it. In the case of fish, we have long had cause for concern about contamination. Smaller and farm-raised fish are potential vehicles for industrial chemicals, such as PCBs. Larger predatory fish, such as tuna, concentrate in their own flesh the mercury they ingest in the flesh of their prey.

And recent studies of omega-3 have mostly disappointed those fishing for evidence of clear benefit to cognition or cardiovascular health.

All of which serves to beg the very question being posed: Is eating fish good for us, in spite of it all?

In my opinion, yes. I'll fillet open my case.

First, there is evidence from an aggregation of available data that while there may be some harm from contaminants in fish, there is still net benefit from eating fish. There is, overall, less disease and premature death when fish is a regular part of the diet than when it is not. Those effects would likely be greater still in a world where fish was pure -- but in this world, perfect is the enemy of good. Eating fish is, demonstrably, good.

But that, in turn, invites another question: good, compared to what? For the most part, studies show benefit when comparing real-world diets with more or less fish intake. Those real-world diets are generally far from optimal. Would adding fish to a high-quality vegetarian or vegan diet confer health benefit? I very much doubt it, although we have no data on which to base a conclusion.

One of the important deficiencies in our research about dietary variations and their health effects is failure to consider how adding one food displaces another, or vice versa. People who eat more fish are either just eating more, overall, or eating more fish and less of something else. What is that something else? We don't know, but my guess is that salmon is more likely to supplant salami than salad. There is evidence linking higher intake of meat to increased risk of chronic disease. If more fish means less meat intake, the apparent benefits may be in part from what people are eating -- and in part from what they are not.

This notion lends some context for interpreting studies of omega-3 fat. Intervention studies of nutrient supplements set the bar rather high; the supplement in question must generate measureable benefit when all else is equal between groups. So, for instance, imagine two groups of cardiac patients, both receiving state-of-the art pharmacotherapy, and with similar patterns of diet, exercise, sleep, and so on. One group gets an omega-3 supplement and one does not. When we hear that omega-3 fat is without decisive cardiac benefit, it is based on such data.

But eating fish doesn't just add omega-3 fat. It changes the pattern of the diet overall. Even fish without omega-3 fat may confer benefit, because it is a lean protein source that displaces other, less nutritious protein sources from the diet. Much of this remains conjectural, but it is reasonable to conjecture that the active ingredient responsible for health benefits from eating fish is... fish.

I believe entirely plant-based diets are among the best options for our health and the health of the planet. Neither I, nor anyone, can say on the basis of good data whether adding fish to such diets enhances their health effects. So the vegetarians reading this may reasonably take it all with a grain of sea salt and throw it back.

Everyone else, however, is encouraged to reel in the take-away: Eating fish confers net benefit. Maybe it's the omega-3, and maybe not. Maybe it's because of adding fish, and maybe it's because of displacing meat. Maybe it's all of these, and it's in spite of the contaminants that come along for the ride. For most of us, two or more fish meals a week -- preferably an as-pure-as-possible, cold water fatty fish such as wild Alaskan salmon -- is a good idea.

For those wondering: Yes, I practice what I'm preaching. I don't eat meat, but I do eat fish as described -- and take an omega-3 supplement as well.